Main efforts in the development of Li-ion systems are concerned with portable electronic devices, like portable phones, camcorders and lap-top computers, and are aimed at increasing the battery power density. Furthermore, there is a need for low cost, low pollution but high specific performance batteries especially in the huge market of electric vehicles (EVs) and hybrid-electric vehicles (HEVs) (see B. Scrosati, Nature 373 (1995), 557, and M. Piana et al., Ionics 8 (2002), 17). In particular, the development of materials for the positive electrode is one of the basic lines of research.
Mixed orthophosphates LiMPO4 (where M=Mn, Fe, Co, Ne) are known in the art (see A. K. Padhi et al., J. Electrochem. Soc. 144 (1997) 1188). These mixed orthophosphates are isostructural to olivine and have been intensively studied as lithium insertion cathode materials for the next generation of Li-ion secondary batteries (see S. Franger et al., J. Power Sources 119-121 (2003), 252). Among these compounds the mineral triphylite, having the formula LiFePO4 and showing an ordered olivine structure, has proven to be one of the most promising among the polyanionic compounds tested over recent years (see e.g. A. Yamada et al., J. Electrochem. Soc. 148 (2001) A 224).
This compound shows several advantages compared with conventional cathode materials such as LiCoO2, LiNiO2 and LiMnO2, namely it is lower in toxicity and relatively inexpensive. In addition, LiFePO4 has an interesting theoretical specific capacity of about 170 mAh g−1, a good cycle stability and a technically attractive flat voltage versus current profile of 3.45 V versus Li+/Li, due to a two-phase extraction/insertion process. A further advantage of this material, thanks to its stability, is the improved safety at high temperatures compared to the transition-metal oxides that lose oxygen on overcharging, which increases the probability of electrolyte decomposition at higher temperatures.
Lithium iron phosphate, at the first charge, can de-intercalate 1 Li+ ion per formula unit, corresponding to the oxidation of Fe2+ to Fe3+. The extraction of Li+ ions gives rise to a new phase, FePO4 (heterosite), which maintains nearly the same structure: a and b lattice constants decrease slightly while c increases (M. Piana, et al., Ionics 8 (2002), 17, and A. K. Padhi, et al., J. B. Goodenough, J. Electrochem. Soc. 144 (1997), 1188). This feature assures that the process is highly reversible and repeatable.
The first investigations on LiFePO4 as electrode material have put in evidence that capacity reached at room temperature is far below the theoretical one. Moreover, a reversible capacity loss is present throughout the charge-discharge cycles, increasing with the current density (A. K. Padhi, J. Electrochem. Soc. 144 (1997), 1188 and A. Yamada, et al., J. Electrochem. Soc. 148 (2001), A224). This capacity loss seems to be related to the limited area of the interface between the LiFePO4 phases where the Li+ extraction/insertion takes place. It is believed that the factor limiting the full conversion of LiFePO4 to FePO4 is based on the combination of low lithium ion diffusion rate and poor electronic conductivity (A. S. Andersson, J. O. Thomas, J. Power Sources 97-98 (2001), 498).
It has been readily recognized that the grain size is a critical issue to minimize high current capacity loss; e.g. 95% of the theoretical capacity at room temperature and at a current density higher than 0.1 mAcm−2 were obtained using samples having 20 μm particle size (A. Yamada, et al., J. Electrochem. Soc. 148 (2001), A224).
Apart from increasing temperature, which can have a positive influence but is impractical for Li-ion batteries directed to a wide market, another possible way of improving LiFePO4 performance is coating the grains with carbon, thus improving the capacity through an increase of conductivity (N. Ravet, et al., in: Proceedings of the ECS Meeting, Abstracts 99-2 (1999), 127, and N. Ravet, Abstract of IMLB-10 (2000), 166) or by using organic materials, like sucrose, added during preparation (N. Ravet, et al., J. Power Sources 97-98 (2001), 503, and S. Yang, P. Y. Zavalij, M. S. Whittingham, Electrochem. Commun. 3 (2001), 505). Further invenstigations were carried out on phospho-olivine compounds using ascorbic acid and citric acid as carbonaceous additives (M. Piana, et al., Ionics 8 (2002), 17, and N. Penazzi, J. Eur. Ceram. Soc. 24 (2004), 1381). Interesting results were obtained by adding fine particles of carbon black during the synthesis (P. P. Prosini, et al., Electrochim. Acta 46 (2001), 3517). The kinetic properties of LiFePO4 can be improved by dispersing copper or silver into the solution during synthesis (F. Croce, et al., Electrochem. Solid State Lett. 5 (3) (2002), A47). The finely dispersed metal powder promoted a reduction of particle size and an increase in the material conductivity. It was also claimed that the electronic conductivity of LiFePO4 could be increased by doping with metals supervalent to Li+ (i.e. M2+, Al3+) (S. Chung, et al., Nat. Mater. 1 (2002), 123).
The next logical step was to try to get an efficient charge transport preparing a homogeneous active material with refined grains size and intimate carbon contact. Higher current density capacities were obtained from a LiFePO4/C composite containing 15% of carbon and a 100-200 nm particle size (H. Huang, et al., Electrochem. Solid State Lett. 4 (10) (2001), A 170). The progress in the design of olivine-type cathodes is illustrated by a cathode material preparation involving an addition of a “disordered conductive carbon” added to the precursor of the material, being 3% the minimum amount, and a subsequent stage of high energy ball milling to get nano-scale homogenized particles (A. Yamada, et al., J. Power Sources 119-121 (2003), 232).
Experience in this field has clearly shown that carbonaceous materials added to the precursors during synthesis have a fundamental importance in increasing the LiFePO4 performance. They can act as reducing agents to avoid the formation of trivalent Fe ions during firing, maintain the particles isolated from each other preventing their coalescence and enhance intra and inter particle conductivity. The choice of the additive is, therefore, of marked importance: it will exert the deep influence previously described only if it can take part in the process itself, like in the synthetic routes followed previously (H. Huang, et al., Electrochem. Solid State Lett. 4 (10) (2001), A 170).
In this context, the kind of synthesis used becomes important too. Initially, the most common way of synthesizing LiFePO4 was the solid-state route (A. Yamada, J. Electrochem. Soc. 148 (2001), A 224, and A. S. Andersson, et al., Electrochem. Solid State Lett. 3 (2) (2000), 66, M. Piana, et al., Ionics 8 (2002), 17, and N. Penazzi, et al., J. Eur. Ceram. Soc. 24 (2004), 1381). Nevertheless, higher performing LiMPO4 (where M=Fe, Mn) materials were obtained via a sol-gel synthetic route (M. Piana, et al., Solid State Ionics 175 (2004), 233). The amorphous precursors used allowed the production of sub-micrometric agglomerates smaller than those prepared via solid-state route and produced a very homogeneous carbon dispersion in the phosphate phase. More recently, hydrothermal preparation has been preferentially chosen for its advantages: quick, easy to perform, low cost in energy and easily scalable. With a solid-state reaction, 3 μm LiFePO4 particles were obtained smaller than the 20 μm LiFePO4 grains described in A. Yamada, et al., J. Electrochem. Soc. 148 (2001), A224.
Recent investigations concern the hydrothermal synthesis of LiFePO4 powders using hexadecyltrimethylammonium bromide (G. Meligrana, et al. Journal of Power Sources 160 (2006), 516-522), which is added during synthesis.
The reference describes the preparation of lithium iron phosphate samples by direct mild hydrothermal synthesis. Starting materials were FeSO4.7H2O, H3PO4, LiOH in the stoichiometric ratio 1:1:3 and hexadecyltrimethylammonium bromide C19H42BrN (CTAB). First of all, a CTAB water solution was prepared, stirring the white powder in distilled water at 35° C. for approximately 30 min in order to completely dissolve it. FeSO4 and H3PO4 water solutions were prepared and mixed together. The resulting solution was then added to the surfactant solution under constant stirring and only in the end, so avoiding the formation of Fe(OH)2 which can be easily oxidized to Fe3+, LiOH was added. The mixture, whose pH ranged between 7.2 and 7.5, was vigorously stirred for 1 min and then quickly transferred in a Teflon-lined stainless steel autoclave and heated at 120° C. for 5 h. The autoclave was then cooled to room temperature and the resulting green precipitate was washed, via a standard procedure to ensure complete elimination of the excess of surfactant, filtered and dried at 40° C. overnight. Heating treatment was carried out in inert atmosphere to avoid the oxidation of Fe2+ to Fe3+: the powders were pre-treated at 200° C. (heating rate of 5.0° C. min−1) and then fired at 600° C. (2.0° C. min−1) in pure N2 for 12 h in order to obtain the crystalline phase and to carbonise the surfactant, so obtaining a carbon film that homogeneously covers the grains.
There remains a need in the art for an improved process for hydrothermal synthesis of LiFePO4 powder having advantageous electro-chemical properties. In particular, there remains a need for LiFePO4 powder having an increased performance at high discharge rates. For example, in the automotive field, batteries not only having a high capacity, but also providing for high discharge rates are desired. However, the prior art does not disclose how to modify hydrothermal synthesis of LiFePO4 in order to provide an electro-chemically active powder having improved performance at high discharge regimes. The invention solves the above-described problems.
The present inventors surprisingly found that the use of an organic surfactant in a mixture of water and co-solvent during hydrothermal synthesis of LiFePO4 results in a powder having improved electro-chemical properties especially at high discharge rates. The present invention is based on this finding.